1. Field of the Invention
The present invention relates generally to sensor arrays for acoustic sensing systems, and more specifically, to an apparatus and method in which sensors are spliced inline in a multiple fiber cable while reducing the length of cable that must be opened up to accommodate the sensors.
2. Description of the Related Art
Arrays of fiber optic interferometric sensors show promise in applications where size, electrical interference, and electromagnetic detection make electronic sensors impractical. Such interferometric sensors are capable of measuring a parameter (i.e., a measurand) with a very high dynamic range (e.g., 120 dB). Optical sensor arrays are formed by connecting a series of sensors using fiber optic lines. If each sensor in an array requires a dedicated fiber to carry the detection signal, the large number of fibers required quickly becomes unwieldy as the number of sensors increases.
Optical couplers are commonly used to couple optical signals from a distribution bus to the sensors, and from the sensors to a return bus. In an array in which amplifiers are not used to periodically boost the signal strength, delivering sufficient optical signal to each of the sensors can be problematic. In addition, there may be significant variations in the optical power levels returned to the detectors, thereby complicating the signal processing.
Optical cable typically includes multiple fiber lines, some of which can be dedicated for distributing optical signals, and some for returning optical signals. Optical cable is not generally sold with sensors already spliced inline, however, since the needs of users with respect to the placement and the number of sensors varies greatly. Thus, sensors are normally spliced inline by the end user.
The splicing of a sensor inline involves opening up the cable by stripping back a portion of the sheath to expose the optical fibers housed therein. A distribution fiber is cut near the midpoint of the exposed, stripped back portion, and both ends of the cut fiber are fused, in turn, to leadlines from distribution ports of a sensor. This same procedure is used for a return fiber, which is coupled to return ports of the sensor. The stripped-back portion may need to be quite extensive to permit sufficiently long working lengths of fiber. For example, a splicing unit for fusing a fiber to a leadline of a sensor may be quite bulky, so that the cut fibers from the cable must extend some distance from the cable to the splicing unit.
After the sensor has been spliced to fibers within the cable, the sensor must be shielded from the environment, i.e., placed within some kind of housing which can be integrated with, and sealed to, the cable. The diameter of this housing is larger than that of the cable itself so that the sensor can reside within it. However, this results in a stiff and bulky cable which can present a problem, for example, when the cable is deployed or retracted, a problem that is exacerbated when the housing is longer.
Thus, there is a need for a technique for splicing fibers inline in a multiple fiber cable in which the length of the region affected by the splicing procedure is reduced.